A plug can "burn out" (break the coil) right after you've started the engine but you'd never know until the next time you tried to start it. A broken coil means no electrical circuit but once the engine is running it doesn't care about electrical continuity, so long as the coil is still there it'll keep right on glowing all through a flight. Back in the *ahem* earlier days we used to take a pin with us so if the coil broke we could dig around inside and get the broken ends back into contact for the next flight .